ewton made good friends, and was helpful in preparing a
treatise on optics for the press. His help is acknowledged by Dr. Barrow
in the preface, which states that he had corrected several errors and
made some capital additions of his own. Thus we see that, although the
chief part of his time was devoted to mathematics, his attention was
already directed to both optics and astronomy. (Kepler, Descartes,
Galileo, all combined some optics with astronomy. Tycho and the old ones
combined alchemy; Newton dabbled in this also.)

Newton reached the age of twenty-three in 1665, the year of the Great
Plague. The plague broke out in Cambridge as well as in London, and the
whole college was sent down. Newton went back to Woolsthorpe, his mind
teeming with ideas, and spent the rest of this year and part of the next
in quiet pondering. Somehow or other he had got hold of the notion of
centrifugal force. It was six years before Huyghens discovered and
published the laws of centrifugal force, but in some quiet way of his
own Newton knew about it and applied the idea to the motion of the
planets.

We can almost follow the course of his thoughts as he brooded and
meditated on the great problem which had taxed so many previous
thinkers,--What makes the planets move round the sun? Kepler had
discovered how they moved, but why did they so move, what urged them?

Even the "how" took a long time--all the time of the Greeks, through
Ptolemy, the Arabs, Copernicus, Tycho: circular motion, epicycles, and
excentrics had been the prevailing theory. Kepler, with his marvellous
industry, had wrested from Tycho's observations the secret of their
orbits. They moved in ellipses with the sun in one focus. Their rate of
description of area, not their speed, was uniform and proportional to
time.

Yes, and a third law, a mysterious law of unintelligible import, had
also yielded itself to his penetrating industry--a law the discovery of
which had given him the keenest delight, and excited an outburst of
rapture--viz. that there was a relation between the distances and the
periodic times of the several planets. The cubes of the distances were
proportional to the squares of the times for the whole system. This law,
first found true for the six primary planets, he had also extended,
after Galileo's discovery, to the four secondary planets, or satellites
of Jupiter (p. 81).

But all this was working in the dark--it was only the first step--this
empirical